Electrochemical Storage of Hydrogen in Carbon Nanotubes

The electrochemical hydrogenation property of carbon nanotubes (CNTs) was investigated. A slurry consisting of CNTs and a binder was pasted on a Ni foam to form an electrode. The discharge capacity was tested in an alkaline electrolyte. The CNTs were prepared by different methods, including chemical vapor deposition (CVD) using different catalysts (Co, Ni, etc.) in an atmosphere of C2H2/N2/H2 at 750oC, arc discharge (AD) using Ni-Y as catalyst under He atmosphere, or laser ablation (LA) using Co or Ni catalyst. All of the CNTs contained various amounts of residual metal catalyst. Carbon black powder (CBP) was also tested. A high discharge capacity of 200 mAh/g of the CVD-Co electrode was obtained, followed by CVD-Ni (50 mAh/g), AD (8 mAh/g) and CBP (8 mAh/g) at room temperature. The CNTs were purified by HNO3 to get a purity higher than 95%, and similar test was conducted. It was found that the discharge capacities were dramatically recuced. For example, the discharge capacity of CVD-Co was reduced to only 32 mAh/g. When the purified CNTs were deposited with a controlled amount of metal on the surface, the discharge capacity was improved significantly. For example, Fig.1 shows the discharge capacities of one sample electrode (CVD-Co0.4Mg0.6) subjected to various treatments. The original (purified) sample exhibits negligible capacity. When it was soaked with 3 and 6.3% Co(No3)2, the discharge capacities were raised to about 25mAh/g. When the salt was reduced to metallic Co, the capacities were further increased to 100 and 170 mAh/g, respectively. It is postulated that the electrochemical hydrogen storage by carbon nanotubes might be influenced by the catalytic effect.